1. Field of the Invention
The present invention relates to a methanol oxidation catalyst and a method for producing the methanol oxidation catalyst.
2. Description of the Related Art
Solid polymer type fuel cells, and particularly solid polymer type fuel cells using an aqueous methanol solution as fuel can work at low temperatures and can be small-sized and reduced in weight. Such a fuel cell converts chemical energy into electric power by the catalytic reaction of an electrode. Therefore, a highly active catalyst is essential to develop a high-performance fuel cell.
PtRu is usually used as the anode catalyst of a fuel cell. The theoretical voltage obtained by the catalytic reaction of the electrode is 1.21V, whereas the voltage loss due to the PtRu catalyst is about 0.3V. Various techniques for improving methanol oxidation activity have been reported.
For example, a method in which metals such as tungsten, tantalum and niobium are added is described in U.S. Pat. No. 3,506,494. The reaction field of a catalytic reaction exists on the surface of catalyst particles having a nano-size and therefore, a few atomic layers on the surface of the catalyst have a large influence on the activity of the catalyst. For this reason, there is the possibility that the state of the surface of the catalyst is changed depending on the synthetic process even in the case where the composition of the catalyst is the same. There is also the possibility that a highly active catalyst over PtRu will be found by controlling the synthetic process to synthesize catalyst particles having a nano-structure that has not been developed so far. A solution method such as a dipping method has been usually adopted to synthesize catalysts.
The solution method involves some difficulties in controlling the structure and surface state of the catalyst, and such a tendency is significant in the case of elements which are resistant to reduction and are hardly alloyed. Because the catalyst is nano-particles, the state of electrons on the surface of the catalyst and the nano-structure of the particles are strongly dependent on the type and amount of the elements to be added. In order to obtain high activity and high stability, it is necessary to appropriately adopt the type and amount of the elements to be added and a combination of these elements.
In U.S. Pat. No. 6,171,721, and Jpn. Pat. Appln. KOKAI Publication Nos. 2004-281177 and 2006-179445, there are descriptions concerning a sputtering process. Besides Pt and Ru, many elements are listed and ternary systems such as Pt—Ru—W and Pt—Ru—Mo are also reported. However, only insufficient studies have bee made as to catalyst compositions, and no catalyst having satisfactory methanol oxidation activity and stability has been established yet.